I take it you've read the reports about the killing of Federal Police officers earlier this month and the escalating war with drug cartels in Michoacan state?
Unless you plan on buying and selling illegal drugs while you're on the highway, you should be safe. It could be argued there's never been a safer time to drive the highway because in response to the killing of the officers the Mexican government sent 5000 troops to the region. This week they arrested 3 members of the La Familia drug cartel allegedly responsible for the killings. Maybe that'll spark some retaliatory move, but unless you're a cop or a drug baron it's extremely unlikely you'll be affected.About 10 million Americans visit Mexico every year and only a handful run into trouble, and usually because they're involved with the drug trade.
The usual rules about driving in Mexico apply: avoid driving at night - make sure you're at your destination before dusk (Ixtapa is only 3 hours from Morelia so that should be no problem). Have a full tank of gas before you leave Morelia as Pemex stations can be hard to find.
Driver training standards in Mexico are not exactly top notch, so drive defensively.
Mexican law states that the owner of a foreign (US) registered car must be driving or IN the car at all times, otherwise it will be confiscated. about 6 years ago
I've just heard from Bill Bell, who with his wife Dorothy runs the Mexico trip planner and road log www.ontheroadin.com. They've done thousands of miles in their RV all over Mexico. Bill's been very helpful, here's what he says about the Siglo XXI:
"Yes we have driven that highway this past Spring. It is an excellent highway and is well patrolled by Federal Police and Green Angels. There are concerns with some Towns along the route but there is no need to stop in the city of Cardenas (you can by pass it.) Of course tell your client to drive only during the day as if there were to be problems it usually happens at night, i.e. animals on the roads." about 6 years ago